The present application relates to an anode for a lithium ion secondary battery that contains an anode active material containing silicon (Si) as an element, a lithium ion secondary battery including the same, an electric tool using the lithium ion secondary battery, a battery car using the lithium ion secondary battery, and an electric power storage system using the lithium ion secondary battery.
In recent years, portable electronic devices such as camera-integrated VTR (videotape recorders), mobile phones, and notebook personal computers have been widely used, and it is strongly demanded to reduce their size and weight and to achieve their long life. Accordingly, as a power source for the portable electronic devices, a battery, in particular a light-weight secondary battery capable of providing a high energy density has been developed. In recent years, it has been considered to apply such a secondary battery not only to the small electronic devices but also to a large electronic device represented by a battery car or the like.
Specially, a secondary battery using insertion and extraction of lithium for charge and discharge reaction (so-called lithium ion secondary battery) is extremely prospective, since such a secondary battery is able to provide a higher energy density compared to a lead battery and a nickel cadmium battery.
The lithium ion secondary battery includes an anode having a structure in which an anode active material layer containing an anode active material is provided on an anode current collector. As the anode active material, a carbon material has been widely used. However, in recent years, as the high performance and the multi functions of the portable electronic devices are developed, further improvement of the battery capacity is demanded. Thus, it has been considered to use silicon instead of the carbon material. Since the theoretical capacity of silicon (4199 mAh/g) is significantly higher than the theoretical capacity of graphite (372 mAh/g), it is expected that the battery capacity is thereby highly improved.
However, in the case where the anode active material layer is formed by depositing silicon as an anode active material by vapor-phase deposition method, the binding characteristics are not sufficient. Thus, if charge and discharge are repeated, there is a possibility that the anode active material layer is intensely expanded and shrunk to be pulverized. If the anode active material layer is pulverized, depending on the pulverization degree, an irreversible lithium oxide is excessively formed resulting from increase of the surface area, and current collectivity is lowered resulting from dropping from the anode current collector. Accordingly, the cycle characteristics as important characteristics of the secondary battery are lowered.
Therefore, to improve the cycle characteristics even when silicon is used as the anode active material, various devices have been invented. Specifically, the technique to form the anode active material layer as a multilayer structure by depositing silicon several times in vapor-phase deposition method has been disclosed (for example, see Japanese Unexamined Patent Application Publication No. 2007-317419). In addition, the technique to cover the surface of the anode active material with a metal such as iron, cobalt, nickel, zinc, and copper (for example, see Japanese Unexamined Patent Application Publication No. 2000-036323), the technique to diffuse a metal element such as copper not being alloyed with lithium in an anode active material (for example, see Japanese Unexamined Patent Application Publication No. 2001-273892), the technique to form a solid solution of copper in an anode active material (for example, see Japanese Unexamined Patent Application Publication No. 2002-289177) and the like have been proposed. In addition, the applicant of the application has disclosed the technique that a multilayer structure in which the first layer and the second layer both containing silicon and each having different oxygen content are alternately layered is provided, and thereby intense expansion and shrinkage of the anode active material layer are inhibited and structural breakage is inhibited (for example, see Japanese Unexamined Patent Application Publication No. 2004-349162).